In a number of industries and endeavors, it is important to be able to track the location of an object. For example, a human body may be tracked for purposes of animation or sports analysis, or an item of equipment may be tracked for security or logistics purposes. Numerous internally and externally referenced systems have been used for such purposes, but there are still many issues to solve in terms of occlusion, lag, resolution, economical feasibility of the required installation, and so on.
Ultra-wideband (UWB) RF positioning is a relatively new positioning technology that is especially useful for indoor applications. Among the more mature applications of UWB are the so-called asset tracking systems in, e.g., health-care or manufacturing. Commercially available systems may consist of a network of time synchronized UWB receivers which track a large number of small, battery powered and inexpensive UWB transmitters. Typically the time synchronization is implemented using a phase synchronization implying that there is one central clock which drives all the others, but that each receiver might have a (constant) clock offset.
Shorter RF pulses generally enable higher spatial resolution, as would be expected. With reference to the Fourier spectrum of pulse trains or sequences, a broader spectrum is required to produce such shorter pulses. Thus UWB technology is able to make use of very short pulses, typically on the order of 1 ns or less, resulting in a very high spatial resolution as well as relatively good robustness to multipath propagation of the RF signals. RF-based positioning technologies can be roughly subdivided into three categories: systems using time delay, systems using angle-of-arrival and systems using signal strength.
With respect to systems that infer position from the time needed for a signal to travel from the transmitter to the receiver, these systems can localize the position of the transmitters by recording at each receiver the time of arrival (TOA) of an RF-signal transmitted by the transmitter and using this TOA data to calculate the position of the transmitter, typically using known methods such as trilateration or multilateration. However, before any such calculations can be performed, the exact position of each receiver in space must be known, and the relative clock offset between each receiver must also be known with great accuracy. In other words, the system must be calibrated before it can be used for tracking the position of the transmitters. In this context, the calibration parameters consist of receiver positions and receiver clock parameters.
Existing calibration methods focus exclusively on the receiver clock parameters and require the receiver positions and the positions of calibration transmitters placed in the measurement space to be surveyed manually prior to carrying out the calibration and prior to use. The surveying of the receiver positions and calibration transmitter positions is typically a labor intensive and time consuming process, prone to measurement errors, and as such is only feasible for permanent setups, typically in logistics applications, where the setup is part of the permanent ICT infrastructure of a building. The receiver positions and calibration transmitter positions can also be derived approximately from floor plans and installation information in a building. However, such estimated receiver positions, as well as any receiver position errors in surveying can have a disproportionate impact on overall system accuracy, depending on the receiver geometry and the transmitter position in-use.
Furthermore, an often overlooked fact is that spatial inaccuracy in the position of the receivers and/or calibration transmitters placed in the measurement space directly causes errors in the clock offset calibration of the receivers, i.e. spatial and temporal calibration parameters are closely linked and can cause non-intuitive errors in the system performance as a whole. Such system performance degradation can be reduced by introducing more receivers in the system installation, and errors can often be reduced by limiting the use of the system to pre-defined areas with better performance, typically a space enclosed by several receivers. However, current calibration methods have no means of making errors caused by erroneous calibration known to the user. Hence the user may only become aware of a need for improved calibration by noticing inadequate system performance during use.
Moreover, certain applications of UWB positioning systems require the system to be portable and easy to move from one location to another (nomadic), such as to assist in motion capture in films, or to enable games and biomechanics uses. Many applications demand easy and fast deployment in the field in un-controlled environments, such as military/firefighters/police and first responder personnel tracking applications, and such applications are currently hampered by inadequate calibration means.
In particular for mobile setups of a UWB positioning system the need for frequent calibration is evident. Notably, since the time calibration accuracy required is so high (in the order of pico seconds) even exchanging cables in the system or movement of the cables used for time synchronization of the receivers can cause a need for re-calibration. Especially, movement of the cables during use can cause small errors in the system calibration while in-use and would ideally be countered by methods providing in-use calibration.
Furthermore, a very important advantage of RF positioning system, in particular UWB based RF positioning systems, is that they do not suffer from occlusion (blockage of line-of-sight) as in the case of a commonly used optical system based on cameras and active or passive markers, since the RF signal can travel through most materials except metals. Thus the receivers in an RF-based positioning system do not need to have (optical) line of sight to the transmitter being tracked, or indeed to each other. However, in calibration methods known in the art, based on manual surveying, typically using laser rangers and/or “total station” survey equipment (electronic theodolite) are based on optical measurement techniques. Hence, the calibration methods known in the art require line-of-sight between the receivers for calibration, eliminating one of the main benefits of using a RF-based positioning system.
The reader is advised that the foregoing background discussion is not intended to survey the prior art, nor is it intended as an inference or admission that any technique, system or methodology discussed herein is known in the art. Rather, this section is intended only to discuss problems considered by the inventors themselves. For a full and accurate understanding of actual prior art, please refer to actual prior art references and documentation.